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Symbols and Acronyms

vacuum breaker   A device for relieving a vacuum or partial vacuum formed in a pipeline, thereby preventing backsiphoning.

vacuum filter  (1) A filter used to accomplish sludge dewatering and consisting of a cylindrical drum mounted on a horizontal axis, covered with filter media, and revolving partially submerged in a dilute sludge mixture. A vacuum is maintained under the media for the larger part of a revolution to extract moisture. The dewatered cake that is formed is scraped off mechanically for disposal. See also vacuum filtration. (2) A diatomaceous earth filter open to the atmosphere and on the inlet side of a pump.

vacuum filtration  A usually continuous filtration operation that is generally accomplished on a rotating cylindrical drum. As the drum rotates, part of its circumference is subject to an internal vacuum that draws sludge to the filter medium and removes water for subsequent treatment. The dewatered sludge cake is released by a scraper.

vacuum pump  (1) A pump for creating a partial vacuum in a closed space. (2) A pump in which water is forced up a pipe by the difference in pressure between the atmosphere and a partial vacuum. (3) An air compressor used in connection with steam condensers and for improving the suction head on other pumps. The compressor takes its suction at low absolute pressure, performs a large number of compressions, and generally discharges at atmospheric pressure.

valence  An integer representing the number of hydrogen atoms with which one atom of an element (or one radical) can combine (negative valence), or the number of hydrogen atoms the atom or radical can displace (positive valence).

valve  (1) A device installed in a pipeline to control the magnitude and direction of the flow. It consists essentially of a shell and a disk or plug fitted to the shell. (2) In a pump, a waterway, passage through which is controlled by a mechanism.

vapor  (1) The gaseous form of any substance. (2) A visible condensation such as fog, mist, or steam that is suspended in air.

vaporization  The process by which a substance such as water changes from the liquid or solid state to the gaseous state.

vapor pressure  (1) Pressure exerted by a vapor in a confined space. It is a function of the temperature. (2) The partial pressure of water vapor in the atmosphere. See also relative humidity. (3) The partial pressure of any liquid.

velocity head  (1) The vertical distance or height through which a body would have to fall freely, under the force of gravity, to acquire the velocity it possesses. It is equal to the square of the velocity divided by twice the acceleration of gravity. (2) The theoretical vertical height through which a liquid body may be raised by its kinetic energy. It is equal to the share of the velocity divided by twice the acceleration caused by gravity.

velocity meter  A vaned water meter that operates on the principle that the vanes of the wheel move at approximately the same velocity as the flowing water.

Venturi meter  A differential meter for measuring the flow of water or other fluid through closed conduits or pipes. It consists of a Venturi tube and one of several proprietary forms of flow-registering devices. The difference in velocity heads between the entrance and the contracted throat is an indication of the rate of flow.

vertical pump  (1) A reciprocating pump in which the piston or plunger moves in a vertical direction. (2) A centrifugal pump in which the pump shaft is in a vertical position.

vertical screw pump  A pump, similar in shape, characteristics, and use to a horizontal screw pump, but which has the axis of its runner in a vertical position.

virus  The smallest (10 to 300 mm in diameter) life form capable of producing infection and diseases in man and animals.

viscosity  The molecular attractions within a fluid that make it resist a tendency to deform under applied forces.

V-notch weir  A triangular weir.

void  A pore or open space in rock or granular material not occupied by solid matter. It may be occupied by air, water, or other gaseous or liquid material. Also called interstice or void space.

volatile  Capable of being evaporated at relatively low temperatures.

volatile acids  Fatty acids containing six or fewer carbon atoms. They are soluble in water and can be steam-distilled at atmospheric pressure. They have pungent odors and are often produced during anaerobic decomposition.

volatile solids (VS)  Materials, generally organic, that can be driven off from a sample by heating, usually to 550 °C (1022 °F); nonvolatile inorganic solids (ash) remain.

volatile suspended solids (VSS)  That fraction of suspended solids, including organic matter and volatile inorganic salts, that will ignite and burn when placed in an electric muffle furnace at 550 °C (1022 °F) for 60 minutes.

volt  The unit of electromotive force or electrical pressure (analogous to water pressure). It is the electromotive force that, if steadily applied to a circuit having a resistance of one ohm, will produce a current of one ampere.

voltage (of a circuit)  The root mean square (effective) difference in potential between any two points of the circuit concerned. On various systems such as a three-phase, 4-wire and a single-phase, 3-wire, there may be various circuits of varying voltages.

volumetric  Pertaining to measurement by volume.

volute pump  A centrifugal pump with a casing made in the form of a spiral or volute as an aid to the partial conversion of the velocity energy into pressure head as the water leaves the impellers.
vortex  A revolving mass of water in which the stream lines are concentric circles and in which the total head for each stream line is the same.